FIG. 4 illustrates an example of conventionally known construction of a steering apparatus for applying a steering angle to steered wheels (front wheels). In this steering apparatus, a cylindrical steering column 2 is supported by a vehicle body 1, and a steering shaft 3 is supported radially inside of the steering column 2 so as to be able to rotate freely. The steering shaft 3 protrudes toward the rear from the opening on the rear end of the steering column 2, and a steering wheel 4 is fastened to the rear-end portion. When the steering wheel 4 is rotated, the rotation is transmitted to an input shaft 8 of a steering-gear unit 7 by way of the steering shaft 3, a universal joint 5a, an intermediate shaft 6, and a universal joint 5b. As the input shaft 8 rotates, a pair of tie rods 9 that are located on both sides of the steering-gear unit 7 are pushed and pulled, and a steering angle is applied to a pair of left and right steered wheels according to the amount that the steering wheel 4 is operated. In the case of the steering apparatus illustrated in FIG. 4 an expandable steering column and steering shaft are employed in order that the forward-backward position of the steering wheel 4 can be adjusted. Of an outer column and inner column of the expandable steering column 2, at least the outer column is formed using a light metal alloy such as an aluminum alloy or thin carbon steel for machine structural use in order to lighten the weight thereof.
In recent years, automobiles have been equipped with various kinds of anti-theft apparatuses as a method for preventing theft of automobiles. One kind of such an anti-theft apparatus is a steering lock apparatus that makes it impossible to operate the steering wheel unless a proper key is used. The construction of such a steering lock apparatus has been disclosed in JP 2004-098788 (A), JP 2004-237973 (A), JP 2006-036106 (A), and JP 2006-036107 (A). FIG. 5 and FIG. 6 illustrate a first example of conventional construction as disclosed in JP 2006-036106 (A).
In this first example of conventional construction, a lock housing 10 that constitute the steering lock apparatus is joined and fastened to part of the steering column 2a by way of a clamp fixture 11 and a pair of bolts 12. The lock housing 10 has a cylindrical housing portion 13, a bottom plate portion 14 that is provided in a state that covers the opening on the base end of the housing portion 13, and a pair of installation flange portions 15 that are provided in a state so as to protrude from the outer-circumferential surface on the base-end portion of the housing portion 13. A through hole (not illustrated) is provided in part of the bottom plate portion 14 of the lock housing 10, and a lock through hole 16 is provided in a portion of part of the steering column 2a that is aligned with the through hole; and these holes pass through from the inside of the lock housing 10 and the radially inside of the steering column 2a. Moreover, in a portion of part of the steering shaft 3a that coincides in the axial direction with the through hole in the bottom plate portion 14 and the lock through hole 16, a plurality of engaging concave portions 17 are provided at a plurality of locations in the circumferential direction thereof, such that the portion has a male spline shape with coarse pitch. Furthermore, a lock unit 18 is fastened to the inside of the lock housing 10, and a guide portion 19 that is provided on the tip-end portion of the lock unit 18 fits inside the lock through hole 16. The lock unit 18 has a lock pin 21 that is moved forward or backward in the radial direction of the steering column 2a by an electric actuator 20.
In this kind of steering lock apparatus, the actuator 20 causes the lock pin 21 to move forward toward the radially inside of the steering column 2a when the ignition key is turned off. An elastic force inward in the radial direction is applied to the lock pin 21 by an elastic member such as a compression coil spring or the like. Therefore, in a state that the lock through hole 16 is aligned with one of the engaging concave portions 17, the tip-end portion of the lock pin 21 engages with the engaging concave portion 17, which prevents rotation of the steering shaft 3a. On the other hand, in a state in which the lock though hole 16 is not aligned with an engaging concave portion 17, the tip-end surface of the lock pin 21 comes in elastic contact with a portion on the outer-circumferential surface of the steering shaft 3a that is between a pair of engaging concave portions 17 that are adjacent in the circumferential direction. As the steering shaft 3a is rotated from this state, the tip-end portion of the lock pin 21 engages with one of the engaging concave portions 17, and prevents rotation of the steering shaft 3a. 
However, in this kind of steering lock apparatus, when a steering wheel 4 (see FIG. 4) is forcibly rotated with the ignition key off, a large force is applied to the engagement portion between the engaging concave portion 17 of the steering shaft 3a and the lock pin 21, and there is a possibility that the steering shaft 3a will be broken.
In a second example of conventional construction illustrated in FIG. 7, instead of forming engaging concave portions around the steering shaft, a key-lock collar 22, having engaging concave portions 17a formed at plural locations around the outer-circumferential surface thereof, is fastened around the middle portion in the axial direction of the steering shaft 3b with enough strength to be able to at least practically prevent a steering angle from being applied to the steered wheels even if the steering shaft 3b is rotated in a state that the ignition key is turned off and the lock pin is engaged with one of the engaging concave portions 17a. In other words, in a state that the ignition key is turned off, the steering shaft 3b will not rotate by an operating force for operating the steering wheel 4 in a normal driving position, however, when the steering wheel 4 is rotated by a force that exceeds a value that is regulated by the key-lock regulation, the steering shaft 3b will rotate with respect to the key-lock collar 22.
In the steering lock apparatus of the second example of conventional construction, even when the steering wheel 4 is forcibly rotated with the ignition key off, the key-lock collar 22 and the steering shaft 3b will not be broken. However, when an attempt is made to forcibly rotate the steering wheel 4 in a state that an engaging concave portion 17a of the key-lock collar 22 and the lock pin 21 are engaged, a large force will be applied from the lock pin 21 to the outer-peripheral edge portion of the lock through hole 16 by way of the guide portion 19 due to a static friction force that is larger than the dynamic friction force, before the steering shaft 3b starts to rotate with respect to the key-lock collar 22. The steering column 2a is made using a material having a low rigidity such as a light metal alloy, so when a large force is applied to the outer-peripheral edge portion of the lock through hole 16, there is a possibility that damage such as cracking will occur in the steering column 2a. Normally, the shape of the lock through hole 16 is a rectangular shape such as illustrated in FIG. 8A, however, in that case, stress is concentrated at the four corners of the outer-peripheral edge portion of the lock through hole 16.
On the other hand, as illustrated in FIG. 8B, construction is proposed in which this kind of stress concentration is reduced by using a Venturi tube shaped lock through hole 16a of which the middle portions of the peripheral edge portions on both sides in the axial direction protrude. However, stress concentration in the circumferential direction of the portions where the corners of the guide portion 19 come in contact as indicated by α in FIG. 8B cannot be avoided. Furthermore, as illustrated in FIG. 8C, in JP 2006-036106 (A) and JP 2006-036107 (A), an oval shaped lock through hole 16b is employed, and in this case, the stress on the peripheral edge portion of the lock through hole is dispersed. However, it is also necessary that the shape of the guide portion 19 of the lock unit 18 be an oval shape, and there is a possibility that installation space will not be able to be ensured, and that the manufacturing cost of the lock unit 18 will increase.
FIG. 9 illustrates another example of a shape of a lock through hole of a steering lock apparatus that is disclosed in JP 2009-190680 (A). In this example, a pair of curved convex portions 23 are provided on the peripheral edge portion on both sides in the circumferential direction of the lock through hole 16c such that the width in the circumferential direction of the lock through hole 16c is the smallest in the middle portion in the axial direction, and becomes larger going in the forward-backward direction. In this case, even when an attempt is made to forcibly rotate the steering wheel 4 with the ignition key off, the tip-end edges of the convex portions 23 where a force is applied from the lock pin 21 by way of the guide portion 19 are curved, so it is possible to disperse that force. However, the steering shaft 3b (see FIG. 7) is such that both the front and rear end portions are supported by both the front and rear end portions of the steering column 2a by way of bearings, and the span of these bearings is long. Therefore, when an attempt is made to forcibly rotate the steering wheel 4 with the ignition key off, it becomes easy for the steering shaft 3b to rotate while bending. In this case, because the tip-end edges of the convex portions 23 are curved, it becomes easy for the guide portion 19 to tilt in the axial direction. When the guide portion 19 is pressed strongly against the peripheral edge portion in the circumferential direction of the lock through hole 16c with the guide portion 19 in a tilted state, there is a possibility that stress that is caused by forces that are applied to the peripheral edge portions of the lock through hole 16c will become concentrated in part of the peripheral edge portion.